Henry J. Snaith's team at the University of Oxford published a research paper titled "Probing ionic conductivity and electric field screening in perovskite solar cells: A novel exploration through ion drift currents" in the journal Energy & Environmental Science. Matthias Diethelm is the first author, and Matthias Diethelm and Henry J. Snaith are co-corresponding authors.

Key Highlights: This paper analyzes the bias-assisted charge extraction (BACE) method under bulk electric field screening and introduces a new BACE-based characterization method, revealing that the initial current density and current decay kinetics depend on ionic conductivity, which is the product of ion density and mobility. Ionic density is independently measured by impedance spectroscopy, and ion mobility is derived from the BACE conductivity. Important differences between low and high ion density cases are highlighted, revealing whether the bulk electric field is fully screened.

In recent years, optoelectronic devices based on metal halide perovskites have attracted widespread attention due to their potential for high efficiency and low-cost production. It is recognized that mobile ions are key material components that affect device response and long-term operational stability. Indeed, the latter can be considered a significant obstacle to the development of competitive photovoltaic technology, which is still dominated by stable silicon solar cells. Electrical characterization methods primarily observe the complex indirect effects of ions on bulk/interface recombination, hindering direct quantification of ion density and mobility in perovskite materials and understanding their impact on bulk and interface electric fields.

To address this issue, Henry J. Snaith's team at the University of Oxford analyzed the bias-assisted charge extraction (BACE) method in the presence of bulk electric field screening and introduced a new BACE-based characterization method, termed ion drift BACE. This method reveals that the initial current density and current decay kinetics depend on the ionic conductivity, which is the product of ion density and mobility. This means that for the unknown high ion densities common in perovskite solar absorber layers, mobility cannot be directly derived from BACE measurements. The researchers derived an analytical model to account for the relationship between current density, conductivity, and bulk field screening, supported by drift-diffusion simulations. By independently measuring the ion density using impedance spectroscopy, they demonstrated how ion mobility can be derived from BACE ionic conductivity. The study highlights important differences between the low and high ion density regimes. For high ion density, the current decay kinetics are independent of the difference between the preconditioning and extraction voltages, while the timescale of the current drop at low ion density strongly depends on the preconditioning voltage. This reveals whether the bulk electric field is fully shielded.

This study establishes BACE as an experimental method for studying mixed ion-electron perovskite materials, going beyond ion density extraction and setting limits and conditions for its use in fully quantifying ion mobility and density in the absorber layer of perovskite solar cells. It elucidates the complex ion-related processes occurring within perovskite solar cells and provides new insights into the operating principles of halide perovskite devices as mixed ion-electron conductors.

Source：DOI: 10.1039/d4ee02494j